If the board is swept with me controlling a Falkenrath Noble, does its ability trigger for each creature that dies or just itself?

Was playing tonight and the situation occured where I had a Falkenrath Noble in play with ~20 other creatures spread across myself and 3 opponents (playing EDH).

I swept the board with a Hour of Reckoning thinking that I would gain 21 life and be able to dole out 21 damage.

This was challenged and I could not find a ruling either way, so I let the challenger have it (I was dominating at this point), taking 1 life and doling 1 life.

Who was correct?